What speed record, still unbeaten for diesel trains, did Britain's InterCity 125 set in 1987?

Britain’s InterCity 125 set a diesel-train speed record of 148 mph (238 km/h) in 1987. The record was achieved during a test run on November 1, 1987, on the East Coast Main Line between Darlington and York.

The train involved an HST formation using British Rail Class 43 power cars. The run tested a new bogie design intended for Mark 4 coaches, rather than being a normal passenger service. The train’s measured peak is also reported as 148.5 mph, so the quiz’s 148 mph figure is the conventional rounded form.

The result extended the HST’s remarkable record-breaking history. A prototype InterCity 125 had already reached 143.2 mph in 1973, while the production HST’s regular service speed was 125 mph. That distinction explains why “125” in the name does not refer to the train’s absolute maximum speed.